Internal combustion engines produce exhaust gases containing a variety of pollutants, including hydrocarbons, carbon monoxide, and nitrogen oxides (“NOx”). Emission control systems, including exhaust gas catalysts, are widely utilized to reduce the amount of these pollutants emitted to atmosphere. A commonly used catalyst for gasoline engine applications is the “three-way catalyst” (TWC). TWCs perform three main functions: (1) oxidation of CO; (2) oxidation of unburnt hydrocarbons; and (3) reduction of NOx to N2.
TWCs, like other exhaust gas catalysts, typically achieve very high efficiencies once they reach their operating temperature (typically, 200° C. and higher). However, these systems are relatively inefficient below their operating temperature (the “cold start” period). As even more stringent national and regional legislation lowers the amount of pollutants that can be emitted from diesel or gasoline engines, reducing emissions during the cold start period is becoming a major challenge. Thus, methods for reducing the level of NOx and hydrocarbons emitted during cold start condition continue to be explored.
For cold start hydrocarbon control, hydrocarbon (HC) traps including zeolites as hydrocarbon trapping components have been investigated. In these systems, the zeolite component adsorbs and stores hydrocarbons during the start-up period and releases the stored hydrocarbons when the exhaust temperature is high enough to desorb hydrocarbons. The desorbed hydrocarbons are subsequently converted by a TWC component either incorporated into the HC trap or by a separate TWC placed downstream of the HC trap.
For instance, U.S. Pat. No. 5,772,972 discloses a hybrid system of hydrocarbon trapping material and palladium based three-way catalyst material. U.S. Pat. No. 6,617,276 teaches a catalyst structure comprising a first layer consisting essentially of a hydrocarbon-adsorbing zeolite and a K, Rb, or Cs active metal that is impregnated on the zeolite, at least one additional layer consisting essentially of at least one platinum group metal, and a catalyst substrate on which the first layer and the one or more additional layers are disposed. In EP 1129774, a hydrocarbon adsorbing member is taught that comprises a zeolite having SiO2:Al2O3 molar ratio of 100 or more and an average primary particle diameter of 1 μm or less of, and that it is free from a monovalent metallic element. U.S. Pat. No. 6,074,973 teaches a catalyzed hydrocarbon trap material comprising palladium and silver dispersed on a high surface area metal oxide support and a zeolite material such as one or more of ZSM-5, Beta, Y, and other suitable zeolites.
U.S. Appl. Pub. No. 2012/0117953 A1 teaches a three way catalyst that comprises an extruded solid body comprising 10-100 weight percent of at least one binder/matrix component, 5-90 weight percent of a zeolitic molecular sieve, a non-zeolitic molecular sieve or a mixture of any two or more thereof, and 0-80 weight percent of an optional stabilized ceria. The catalyst comprises at least one precious metal and optionally at least one non-precious metal, wherein: (i) the at least one precious metal is carried in one or more coating layer(s) on a surface of the extruded solid body; (ii) at least one metal is present throughout the extruded solid body and at least one precious metal is also carried in one or more coating layer(s) on a surface of the extruded solid body; or (iii) at least one metal is present throughout the extruded solid body, is present in a higher concentration at a surface of the extruded solid body and at least one precious metal is also carried in one or more coating layer(s) on the surface of the extruded solid body. In addition, U.S. Appl. Pub. No. 2012/0308439 A1 teaches a cold start catalyst that comprises (1) a zeolite catalyst comprising a base metal, a noble metal, and a zeolite, and (2) a supported platinum group metal catalyst comprising one or more platinum group metals and one or more inorganic oxide carriers.
As with any automotive system and process, it is desirable to attain still further improvements in exhaust gas treatment systems, particularly under cold start conditions. We have discovered a new three-way catalyst that provides enhanced cleaning of the exhaust gases from internal combustion engines.